The piezoelectric converter is used as a piezoelectric inkjet head for discharging ink droplets and carry out printing in an on-demand type inkjet printer.
The piezoelectric converter has such a structure, as described in Japanese Unexamined Patent Publication No. JP-H11-34320-A2 (1999), that a piezoelectric actuator 92 that includes an electrically conductive oscillator 921 of such a size that covers a plurality of cavities 911, a piezoelectric ceramic layer 922 of flat plate shape having such a size that covers the plurality of cavities 911, and a plurality of individual electrodes 923 which are separated in correspondence to the cavities 911 and having a size corresponding to each of the cavities 911, which are stacked on one side of a plate-shaped substrate 91 that has the plurality of cavities 911 to be filled with ink disposed in the direction of plane (refer to FIG. 3).
The electrically conductive oscillator 921, together with the individual electrodes 923, sandwiches the piezoelectric ceramic layer 922 so as to serve also as a common electrode for applying a drive voltage to the piezoelectric ceramic layer 922.
The substrate 91 usually has a form of plate made of a metal such as stainless steel. Each cavity 911 is connected via a nozzle passage 912 to a nozzle 913 that reaches the surface of the substrate 91 opposite to the side where the piezoelectric actuator 92 is stacked, for discharging ink droplets. Each cavity 911 is also connected via a feed port to a common feed passage that supplies the ink from an ink tank of the inkjet printer (not shown).
When a drive voltage is applied between the oscillator plate 921 serving as the common electrode and at least one of the plurality of individual electrodes 923, while the cavities 911 are filled with ink, a region of the piezoelectric ceramic layer 922 where the drive voltage is applied contracts in the direction of plane. Since the piezoelectric ceramic layer 922 is fastened onto the oscillator plate 921, the region of the piezoelectric actuator 92 where the drive voltage is applied deflects so as to protrude toward the cavity 911 in accordance to the contraction. This deflection compresses the ink in the cavity 911, so that an ink droplet is discharged through the nozzle 913 for printing.
The piezoelectric converter shown in FIG. 3 is manufactured by placing the substrate 91 having a plurality of recesses that would become the cavities 911 formed on one side thereof and the piezoelectric actuator 92 having the laminated structure as described above one on another via a thermosetting adhesive layer (not shown), and heating the stack while applying a pressure in a direction perpendicular to the surface so as to harden the adhesive and hold the stack together.
In the piezoelectric converter of the prior art, however, when cooled down to the room temperature after bonding, significant buckling deformation (deflection) tends to occur in a free region of the piezoelectric actuator 92 which corresponds to the cavity 911 and is not fastened onto the substrate 91, namely a region that deflects so as to protrude toward the cavity 911 when the drive voltage is applied thereto. This buckling deformation impedes the region from deflecting when the drive voltage is applied thereto, and there was such a problem that the ink droplet cannot be properly discharged through the nozzle 913.
The problem described above is caused by thermal stress due to the difference in thermal expansion coefficient between the metal that makes the substrate 91 and the piezoelectric ceramic material that makes the piezoelectric ceramic layer 922.
Since metals have generally higher thermal expansion coefficients than ceramics, when the adhesive is heated so as to bond the substrate 91 and the piezoelectric actuator 92 together through thermosetting of the adhesive, the substrate 91 made of a metal undergoes larger thermal expansion in the direction of plane than the piezoelectric actuator 92 that includes the piezoelectric ceramic layer 922, in the early stage of heating when the adhesive has not yet hardened.
As the adhesive hardens and both members are fastened together in this state, the substrate 91 is subjected to larger contraction in the direction of plane than the piezoelectric actuator 92 in the cooling process, resulting in the stress concentration in the direction of plane in the region of the piezoelectric actuator 92 that corresponds to the cavity 911, which causes a significant buckling deformation in the region.